-- o ICOM

Numbef" 20 on your Feedback c.rd
CW Transceiver for 20 Meters
Get on this favorite OX band.
Adapted from G-DRP Club Circuit Handbook
A
re you ready for the challenge of 20m
QRP? Want the added enjoyment of
rolli ng your own rig? If so on both COUnlS ,
then check out the following home-brew project!
This interesting little project uses componcnrs
ranging from tubes to ICs.
Hybrt d
Some of the modules used to build this rig
were removed from existing equ ipment ; others
were built specially. This explains the
different devices for the TX modulator and
the RX mixer, the option of two carrier injection
oscillators (CIO) , and the SSB-type filter
instead of a CW filter.
Clrcun Descriplion
The Colpitts VFO circuit, Figure I, has
gate clamping to improve stability . It' s followed
by two buffers , the second providing
by Frank Lee G3YCC
individual outputs to the TX and RX. A RIT
circuit operates on receive. You must set it up
correctly when you fi rst align the rig (see
instructions below).
Build the VFO in a stout metal box, with
care for mechanical rigidity. The RX front
e nd uses 40673 dual -gate MOSFETs (see
Figure 2). The tuned circuits are peaked to
the center of the CW band with trimmers.
Coil winding details for the VFO and other
stages appear in the respective figures.
The mixer output is link-coupled to a KVO
9 MHz SSB filter , whose output is amplified
by an SL612 IF amplifier IC (see Figure 3).
The product detector uses two BC 107 tran ­
sistors. Carrier reinjection is from a crystal
oscillator using the USB crystal supplied with
the KVG filter. Figure 4 contains two diagrams:
an FET oscillator and a PNP bipolar
oscillator. You can use either, or you can
alter the circuit pola rities to suit an NPN
transistor. An LM380 Ie would work fine
he".
TheTX mixer is an MOIOS (Figure 5), fed
from the VFO and the lowe r sideband carrier
injectio n oscillator. (You could probably usc
the less expensive 40673 as a mixer.) The
mixer output is applied to an SL6IOC RF
amplifie r. then to a FET which drives a
12BY7A PA similar to that in the well-know n
" Tucker Tin,. circuit. I tried to use a transistor
PA, but had troubles with it (likely due to
my lack of experience in transistor PA
design!).
A simple diode/meter circuit measures
relative power output (see Figure 6). Sidetone
is provided by an NE555 circuit (see
Figure 7).
Send/receive sw itching is by means of a
a-pole change-o ver relay controlled from a
Ll • 30T_. 34 SWG
ON 1/4,n. 0 SLUG -TUNED
FORMER
" 0 .,
~6 _8 V
l!l < 5.1 MHt j ZEN
"
4 ~ ~p F
"
" ~ 30 p F
,F
+12 V
4
REG.
" 1°' 1°'
r. BCIDa
~ IN9 14
2N 38 19
BC I Qa
'"
"
'OK V" 0I,5C
; :::: ~ 8 0
"
, r. 0.01
~
0,01
I
,I Rx MIX
-
DI SC
RFe '"
I
,I
T. MIX
""lOOT 30S WG
T68 0PF
470
ON IW RE S.
3.3K
Figure J. Colpitts VFO c;rcuit.
L I, L 2' 251. 30 5WG,
ON 1/4 i n
SLUG' TUN ED.
LI NKS 4 T.
L3' 30T. 30 SWG
1/ 4 ill. 0 FORMER.
Utfl( 4 T
1501(
VFO
" 47'F+
4 0 6 73 4 0 6 73
"
0
,,;;r
S
"
ee0
L>
+ 1 2 V~(RI I
47pF
r '-F STR"
• 150 1<
"
* 25 PF *IOnF * IOOnF
ANT, .....,
-::~ 25 pF
::::; tOOnF 150K
RLA ,
'"
" 0
lOOK
2.71<
; ~ 2 0nF
Figure 2. The receive from-end. The associated tuned circuits are pealed to the center o/the CWband ""ith trimmers.
73AmateurRadio • June, 1989 65